This invention relates to improvement in an image formation apparatus, such as an electrophotographic printer or copier, particularly including a plurality of image formation units disposed along a transfer material transport passage for successively transferring toner images to a transfer material moving on the transfer material transport passage.
Known as a conventional image formation apparatus is an apparatus called tandem type including a plurality of image formation units disposed on a transfer material transport passage extending in a horizontal direction, for example, for successively transferring toner images from the image formation units to a transfer material moving along the transfer material transport passage and forming a color image on the transfer material.
The image formation unit refers to a pair of a photoconductor unit having a photoconductor on which an electrostatic latent image is formed and a developing unit for storing toner supplied to the photoconductor. Already proposed as the transport technique is a transfer roll transport technique wherein each image formation unit is provided with a transfer roll for abutting the photoconductor and paper as a transfer material is transported by the photoconductor and the transfer roll, or a belt transport technique wherein paper is, for example, electrostatically attracted and held on a circulating transport belt.
As for the arrangement structure of the image formation units, already proposed are a landscape orientation type wherein a plurality of image formation units are placed transversely side by side relative to a transfer material transport passage extending in the horizontal direction and a portrait orientation type wherein a plurality of image formation units are placed longitudinally relative to a transfer material transport passage extending in a vertical direction.
However, in this kind of the conventional landscape orientation type, often the image formation units are attached and detached from the direction parallel with the transport face of the transfer material transport member and vertical to the transport direction. In this case, the image formation units are positioned in the apparatus main unit by an image formation unit drive member attached to one side of the apparatus main unit and a positioning member formed on an opposite side of the apparatus main unit with the transport member between.
The image formation unit itself is positioned by a positioning section formed in a support member for supporting the photoconductor without directly positioning the photoconductor as the positioning reference on the configuration. Thus, it is difficult to ensure the positioning accuracy of each image formation unit in the apparatus main unit.
As for the conventional image formation apparatus of the portrait orientation type, each image formation unit can be attached and detached from the direction orthogonal to the transfer material transport passage of roughly vertical portion, so that each image formation unit can be positioned in the apparatus main unit by a unit positioning section formed on both sides of a cabinet and it becomes easy to ensure the positioning accuracy; in contrast, however, a disadvantage occurs in the transfer material transportability.
In the transfer roll transport technique, if the image formation unit spacing is wide to some extent, paper passes through the transfer part of one image formation unit, the pass-through paper portion becomes long, the tip state of the paper becomes easily unstable in such a manner that the tip of the paper curls or remains straight, and the tip position of the paper arriving at the transfer part of the next image formation unit easily varies.
Thus, the write start position of each color component toner image relative to the paper at the transfer part of each image formation unit shifts, causing a color shift or color unevenness phenomenon of a color image.
In the belt transport technique, paper is transported on the paper transport belt and thus the tip entry position of paper in the transfer part of each image formation unit is stable and the color unevenness of a color image relative to the paper transport direction can be suppressed as compared with the transfer roll transport technique. However, as the image formation unit spacing is wider, a walk phenomenon in which when the paper transport belt moves, it meanders in the width direction increases, and color shift or color unevenness of color image worsens in the orthogonal direction (width direction) to the paper transport direction.